Properdin is believed to be one of the early acting components of an alternate pathway of complement activation. This system is activated by zymosa (a polysaccharide isolated from yeast cell walls) lipopolysaccharides, and certain dextrans. Native properdin or properdin that has not reacted to form a complex with zymosan, has been purified during the past year by column chromatography (DEAE cellulose, CM sephadex and QAE sephadex) and electrophoresis on polyacrylamide gels at pH 5.0. In addition, we have identified two new components of this alternate complement pathway. They are serum proteins and they are bound by DEAE cellulose, but they have not been purified and their mode of action has not been determined. During the next year, native human properdin will be purified and its physicochemical properties will be compared to those of properdin isolated after it has reacted with zymosan. In addition, the two new components of this system will be further purifed and their reactions with zymosan and properdin will be investigated.